So just to get the negatives out the way.
- The bosses remain to be the weakest part of Metroid. None of them are terrible, just annoying for the most part.
- Zero Mission has a weaker atmosphere than both Super and Fusion.
- Could have done without the statues guiding the player, but whatever. Not a deal-breaker.
- Overall not as good as Super (but that's a pretty high bar to clear anyway)
That aside...

Holy shit. Banger game. The last bit bumps this up a half star.

Super, Fusion and Metroid II (from what I can tell) have their own unique core qualities that are intrinsic to those experiences, setting them apart from each other. Other than the extra segment at the end, Zero Mission doesn't really have anything like this, but that's alright. It's a solid middle ground entry, not too cryptic nor restrictive, and probably the best starting place for a newcomer to the series. Even with how straightforward it might appear, it's still super fun to blast through, and none of the series' trademark cleverness is lost.

EDIT: Actually, I suppose its straightforwardness would be the core quality lol.